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The stress intensity factor is important for understanding crack initiationand propagation. Because it cannot be measured directly, the characterizationof the stress intensity factor relies on the measurement of deformation arounda crack tip. Such measurements are challenging for dynamic fracture of brittlematerials where the deformation is small and the crack tip velocity can be high(>1 km/s). Digital gradient sensing (DGS) is capable of full-field measurementof surface deformation with sub-microsecond temporal resolution, but it islimited to centimeter-scale specimens and has a spatial resolution of only$\sim 1$mm. This limits its ability to measure deformations close to the cracktip. Here, we demonstrate the potential of Shack-Hartmann wavefront sensing(SHWFS), as an alternative to DGS, for measuring surface deformation duringdynamic brittle fracture of millimeter-scale specimens. Using an commercialglass ceramic as an example material, we demonstrate the capability of SHWFS tomeasure the surface slope evolution induced by a propagating crack onmillimeter-scale specimens with a micrometer-scale spatial resolution and asub-microsecond temporal resolution. The SHWFS apparatus has the additionaladvantage of being physically more compact than a typical DGS apparatus. Weverify our SHWFS measurements by comparing them with analytical predictions andphase-field simulations of the surface slope around a crack tip. Then, fittingthe surface slope measurements to the asymptotic crack-tip field solution, weextract the evolution of the apparent stress intensity factor associated withthe propagating crack tip. We conclude by discussing potential futureenhancements of this technique and how its compactness could enable theintegration with other characterization techniques including x-rayphase-contrast imaging (XPCI) toward a multi-modal characterization.

Shack-Hartmann wavefront sensing: A new approach to time-resolved measurement of stress intensity during dynamic fracture of small brittle specimens